The effect of perforation on the wake of a thin flat plate placed normal to the free stream at Reynolds number (
$Re$) 250 (based on plate width
$d$, and inflow velocity
$U_o$) is studied by means of direct numerical simulation. The perforated plate of length
$6d$ consist of six equidistant square holes of varying sizes corresponding to porosity
$\beta$ (ratio of open area to total plate area) of 0 %, 4 %, 9 %, 12.25 %, 16 %, 20.25 % and 25 %. It is observed that the bleed or jet flow through perforations pushes the shear layer interaction farther downstream with increasing
$\beta$. This causes a monotonic decrease in the drag coefficient with increasing porosity, and a sharp fall seeming to begin at
$\beta \approx 4\,\%$. On the other hand, the Strouhal number increases with
$\beta$ up to 16 % (at
$\beta =16\,\%$, loss of flow three-dimensionality leads to a ‘quasi-laminar’ state of flow). This is followed by a sharp fall in the Strouhal number at
$\beta \approx 20\,\%$. The behaviour of the large-scale vortical structures in the far wake is influenced by the near-wake behaviour of the bleed flow, where the local
$Re$ based on the perforation hole size determines the overall flow three-dimensionality. It is also observed that the jet or bleed flow undergoes meandering instability when pitch separation is equivalent to the hole size (at
$\beta =25\,\%$). The low-
$Re$ turbulent flow for a non-perforated plate is altered to a transitional state by the presence of perforation. The streamwise vortex pairs (secondary instabilities) become fairly organized as
$\beta$ is increased from 0 % to 16 %. The secondary instability at
$\beta =16\,\%$ appears similar to mode-B with wavelength
${\approx }1d$. On the contrary, the secondary instability at
$\beta =25\,\%$ appears similar to mode-A with a wavelength of
${\approx }2d$.